A scout for the Arizona Diamondbacks was aware that a well-regarded prospect from the Dominican Republic had used a phony name and age when he signed with the club two years ago, said a former agent for the pitcher currently known as Adriano Rosario.

Rob Plummer, who represented Rosario for a year until late 2001, told ESPN.com that Diamondbacks scout Rafael Mena tried to profit from the deception. The agent said he was in a meeting with Rosario and Mena in the player's hometown of San Francisco de Macoris when Mena threatened to reveal Rosario's real name unless he got a cut from any eventual signing bonus the player might receive.

"I definitely witnessed an Arizona Diamondbacks scout threaten a player about his identity," Plummer said.

The real age of Rosario, whose bio with the Diamondbacks claims he turned 19 on Sunday, has been the subject of rumors in Dominican baseball circles since before he signed with Arizona in June 2002 for a contract that included a $400,000 bonus. Those suspicions were confirmed by a report Sunday that claimed the pitcher, now one of the top prospects in the Arizona system, is using the name of his nephew.

Birth records show that Rosario's real name is Ramon Antonio Pena Paulino, and that he was born Jan. 9, 1982 -- making him 22 years old, not 19, as listed in his player bio with the Diamondbacks, according to documents obtained by The Associated Press. The commissioner's office of Major League Baseball has been investigating the identity of the player as part of a larger inquiry that the Diamondbacks may have violated rules in the acquisition of the pitcher.

Scott Boras, the player's current agent, confirmed to ESPN.com Monday that his client's name is not really Adriano Rosario. Boras, who began representing him late last year, said he discovered the false identity in recent weeks and notified baseball officials of the deception.

A right-hander with a fastball in the upper 90s, Rosario started the year with the team's Double-A affiliate, the El Paso Diablos, but recently left the country. On Thursday, the Diamondbacks assigned his rights to the team's Dominican-based summer league team and issued a statement that read, "Rosario has returned to his native Dominican Republic to address his immigration status."

All Dominican players need U.S. visas, with legitimate personal information, to play in the U.S.

The Rosario signing was already being scrutinized by the commissioner's office because of MLB rules that the team may have broken in the signing of the player. The inquiry was sparked by an ESPN.com report disclosing that the Diamondbacks had paid $100,000 directly to Rosario's agent at the time, Ivan Noboa, as incentive to close the deal.

Noboa is the younger brother of Junior Noboa, who directs the Diamondbacks' Latin American scouting operation and runs the team's Dominican academy. MLB rules prohibit the payment of fees from a club to family members of club employees, as well as agents, for the recruitment of players.

Plummer said he dumped Rosario as a client because the player refused to use his real name.

"The last thing that I wanted for my credibility or that of my player was for a team to find out that's not his real name or age," he said. "It just makes my job five times harder."

But Rosario persisted in the fraud in part because, Plummer said, he was influenced by Mena, a well-known baseball figure in the San Francisco de Macoris area. Posing as a younger player enhanced the propect's value to teams that he was trying out with at the time.

Rosario later told Plummer that Mena had helped him originally to change his identity, according to the agent.

"(Rosario) didn't want to do anything without getting Rafael's stamp of approval," Plummer said. "Rafael had this 'I'm-above-the-law' attitude like he'd never get caught."

Plummer, who is based in New York, has represented several Dominican players who have received seven-figure bonuses, including Los Angeles Dodgers' prospect Joel Guzman, owner of a record $2.25 million bonus, set in 2001. He was also the agent who blew the whistle in 1997 on the Florida Marlins for signing underage pitcher Ricardo Aramboles, who was 14 when he signed for $5,500.

A source close to the pitcher known as Rosario insisted the player fired Plummer, not the other way around, because Ivan Noboa offered better service. But Plummer's account of Mena creating the use of fake identity is supported by Boras, who said Mena served as the player's buscone, a term used in the Dominican to describe anyone who trains unsigned players or brokers them to teams.

"Mena told him that if he used the (phony) name it would help him get more money from a team," Boras said. "These are kids. They'll do anything the buscone tells them to do because they control these kids' lives."

It is unknown whether any other Diamondbacks scouts or officials knew about the ruse involving Rosario's identity. Joe Garagiola Jr., general manager of the Diamondbacks, declined comment on the case other than to say that "as far as I'm aware, the club didn't know" about Rosario using a false name. Junior Noboa said he just learned about the deception but otherwise declined comment, citing instructions from team attorneys.

Rafael Perez, senior manager for MLB's Latin American office in Santo Domingo, said that the Diamondbacks and Major League Baseball conducted separate investigations to confirm the identity of Rosario when he signed. His office approved Rosario's paperwork, despite having learned that people who know the player do not call him Adriano.

"There were clues that should have told us to go forward," said Perez, whose office at the time had just started to do more rigorous checks of player identities and ages due to visa concerns related to the terrorist attacks of 9-11. "One of those clues was that he was called 'Tony.' "

The frequent use of the name -- a short form of the name Antonio -- sparked Plummer's concerns at the time. After meeting the player's supposed mother, who was much lighter-skinned, the pitcher confessed to him that his real name was Tony Pena, Plummer said.

In an interview with ESPN in March during spring training, Rosario denied that he had used a fake name, claiming that "Tony" was just a nickname. When asked in general why some Dominican players use false personal information, he said, "Some Dominicans lie about their age because if they are over 20 years of age, teams don't want to try them out. It's not like here in the U.S. where the teams will sign players who are 25 or 26 years old."

MLB rules restrict the number of roster spots in U.S.-based rookie leagues for players over the age of 20. The younger the Dominican prospects, the more time they have to improve to the point where they can win one of those spots -- as well as a U.S. visa.

In exchange for keeping quiet about Rosario's identity, Mena wanted between $20,000 and $40,000 of his signing bonus, Plummer said.

"I see (Rosario) as an unfortunate pawn caught between money-hungry people who were just trying to take advantage of the kid," he said.

Ivan Noboa said he is familiar with Mena, who also runs a youth league in San Francisco de Macoris. But, he said, he had shared none of the money he received from the signing with Mena. Besides the $100,000 side payment from the team, Ivan Noboa got 25 percent of Rosario's bonus for another $100,000.

Boras said he does not know if Mena received any cut of Rosario's bonus.

The Diamondbacks argue that they broke no MLB rules with the payment to Ivan Noboa, who at the time was also negotiating with other teams for Rosario and might have been able to get more money for him from the Dodgers. After signing with the Diamondbacks, Rosario hired a third agent, Boras, who in recent weeks has been exploring ways to have the player's contract with the club voided, or at least revised to reflect his value in the marketplace.

Mena is no longer employed by the team, Garagiola said. Mena was a Diamondbacks scout until last year.

Baseball America ranks Rosario as the second-best prospect, and No. 5 overall, in the Diamondbacks system. Garagiola has described him as having a future as bright as any player in the team's minor-league system.

Ivan Noboa claims that news that Rosario is using a fake name shocked him. "I thought he was Adriano Rosario," said the agent, who met the player as a teenager. "For three years, I didn't know."

Perez declined to speculate on what action, if any, the commissioner's office might take against the Diamondbacks if, as Plummer alleges, a club employee knew about or participated in the disguising of a player's identity. "That's something I have to mention to the New York office to see where we want to go on this," he said.

There is precedent for heavy penalties for teams involved in deception. In 2000, two Dodgers officials were suspended, the club was fined $200,000 and team's Dominican academy was shut down for a year because a scout altered the birth certificate of Adrian Beltre. The scout used Wite-Out to make the 15-year-old appear 16.

Tom Farrey is a senior writer with ESPN.com. He can be reached at tom.farrey@espn3.com.